The present invention relates to a poly vinyl chloride-free plastisol which is particularly well suited for use in spot seam sealing, especially in motor vehicle construction.
The term plastisol was originally introduced to designate poly vinyl chloride pastes in the form of substantially intimate dispersions of poly vinyl chloride in a plasticiser, in which there could also be certain additives such as extenders, stabilisers, fillers, surface tension-changing materials and diluents. These poly vinyl chloride plastisols are used particularly for the production of synthetic leathers, floor coverings, coatings, conveyor belts and for rotational casting.
However, because of the hydrochloric acid content, which in certain conditions may be released from plastisols of this type based on poly vinyl chloride, the use of these plastisols is limited. Thus they are not suitable as sealing materials for the seam sealing of welded joints, in particular as seam seals in the spot welding of automobile bodies, since, during welding, temperatures are reached at which the poly vinyl chloride decomposes and this leads to corrosion of the welded parts.
However, sealing of the so-called spot seam (the welding is punctiform) is of considerable importance for protection against corrosion.
Therefore a series of synthetic pastes were developed which were called plastisols because of their basic properties and their usage, although they did not contain poly vinyl chloride. The present invention relates to a poly vinyl chloride-free plastisol of this type.
Plastisols of this type are known from DE-OS 1,479,491, U.S. Pat. No. 2,872,429, DE-PS 2,529,732 and DE-PS 2,454,235. None of these known plastisols fulfills the requirements of a plastisol for use in spot sealing.
The plastisols from DE-OS 1,479,491 and U.S. Pat. No. 2,872,429 do not have sufficient storage stability for use in motor vehicle construction.
The poly vinyl chloride-free plastisols known from DE-PS 2,454,235 and DE-PS 2,529,732 certainly have a satisfactory storage stability but have other deficiencies as regards their use for spot seam sealing. Thus, the plastisol according to DE-PS 2,454,235 does not have sufficient adhesive strength for such a use. Certain improvements in the adhesion can be achieved according to DE-PS 2,529,732 where heterocyclic compounds, which contain at least one N atom in the ring, are polymerised as adhesion promoters. However the technical properties thus achieved are not sufficient for use as spot seam sealing. Thus the adhesion promoters based on heterocyclic vinyl compounds according to DE-PS 2,529,732 lead to relatively brittle products, especially when they are used together with acrylic--or methacrylic acid esters, and these products have unsatisfactory properties when subjected to the dynamic stresses (loads) which occur in use. The above mentioned compounds are also not suitable for extensive use in motor vehicle construction because of their annoying smell.
One particular disadvantage of the known poly vinyl chloride-free plastisols according to DE-PS 2,529,732 relates to the elasticity, at low temperatures, of a seam seal produced from a plastisol of this type (rod bending test at -30.degree. C.). When using a plastisol as a spot seam seal it is crucial that a good adhesion is obtained not only with respect to smooth sheet metal but also with respect to sheet metals which are coated with zinc-rich paint. The plastisols from DE-PS 2,529,732 partially solubilise coatings of this type and the adhesion then becomes unsatisfactory.
Plastisol layer thicknesses up to 5mm are generally required for a spot seal. With coatings of this thickness there is a special requirement as regards the flexibility of the seal. Moreover, a satisfactory spot seam seal must have sufficient electrical conductivity. However, the addition of graphite to increase the conductivity in a plastisol of the conventional type has a deleterious effect in so far as rigidity and elasticity are concerned.